Urin

Japanese name:

ウリン

While the people of the sea doesn't share biological bonds the same way the sky dwellers (aka "Human beings") do, Marin still considers Urin to be her little sister. When the ring drops into the sea and Marin decides to head up there and return it, Urin follows her, concerned about what she heard about the sky dwellers.

Later, she accidentally (and partially through manipulation) frees one half of the entity known as Sedna.

SpoilerSomewhere past the halfway point, during a full solar eclipse, Urin becomes the vessel of Sedna, which marks the first time in history of Sedna taking physical form. It's unclear about just how much in control of herself Urin is at this point, but at the end -- after Sedna/Urin takes Marin and Konan's priestess powers away, and we learn the true identity of Sedna -- Urin finally tries, and succeeds, at freeing herself completely.